To enable Clojure to be relicensed under a different open-source license if that would be advantageous.

Signing the Contributor Agreement grants Rich Hickey joint ownership of your contributions. In exchange, Rich Hickey guarantees that Clojure will always be available under an open-source license approved by either the Free Software Foundation or the Open Source Initiative.

Why does Clojure require that contributors submit a contributor agreement on paper?